In automation systems, Ethernet-based field-bus systems are being used more and more. Based on the Ethernet protocol, these systems use an Ethernet data frame for responding to a connected device. However, an Ethernet data frame features not-insignificant overhead to the data, so that, for example, a simple digital field device with, for example, only 8 digital outputs must receive a data frame that is composed of at least 72 bytes. If many field devices that require only very little user data are used in an Ethernet-based system, the Ethernet protocol is very inefficient and makes the system slow.
Another disadvantage of an Ethernet-based field-bus system is the requirement that each node must be physically equipped with an Ethernet interface, despite the fact that often only small distances must be bridged. This leads to unnecessarily high costs. Another disadvantage of the Ethernet interface is that the conversion into digital signals generates a not-insignificant delay, which must be taken into consideration when there are many nodes in a system. Therefore, Ethernet-based field-bus systems can be used only to a limited extent for real-time applications in the field of automation systems.
For the execution of control tasks in real time, it is known, from EP 1 590 927 B1, to connect the nodes for the real-time application in a ring-shaped transmission path, wherein this ring-shaped transmission path is connected via a network coupler to a network over which Ethernet telegrams can be transmitted. The network coupler is here designed so that an Ethernet telegram received from the network via an external interface of the network coupler is forwarded to an internal interface of the network coupler and output over the ring-shaped transmission path, wherein, when the Ethernet telegram circulates over the ring-shaped transmission path, each node connected to the ring-shaped transmission path exchanges the user data designated for each node with the Ethernet telegram as it circulates over the ring-shaped transmission path. Real-time requirements could be maintained by reducing the response time of the individual nodes through the processing of the Ethernet telegrams on the ring-shaped transmission path in the passage through the nodes connected to this transmission path.
However, one disadvantage of the method described in EP 1 590 927 B1 is that the Ethernet telegram circulating on the ring-shaped transmission path has at least 72 bytes and thus features the overhead to the data described above. Furthermore, the use of the ring-shaped transmission path leads to jitter that limits the ability to maintain real-time requirements.